The Lymphatic System: A Shadow Circulation of the Blood Help

By — McGraw-Hill Professional
Updated on Aug 30, 2011

Introduction to The Lymphatic System: A Shadow Circulation of the Blood

Closely associated with immunity is the lymphatic (lim- FAT -ik) system. The lymphatic system consists of a collection of lymphatic organs and lymphatic vessels running between them. In a practical sense, the lymphatic system can be thought of as a shadow circulation of the blood circulation. The main reason is that the lymphatic capillaries run side-by-side (like a shadow) along the tiny blood capillaries (Figure 17.1).

The lymphatic capillaries are special in that they are dead-ended. Since they are closed at the far end, their fluid contents, the lymph ( LIMPF ), flows in one direction only – towards the heart. The word, lymph, literally means “clear spring water.” This meaning derives from the usually clear, watery appearance of the lymph. The lymph is actually a filtrate ( FILL -trait) – filtration product – of the blood, the blood in the nearby capillaries. The blood pressure pushes outward, thereby filtering water, NaCl, and various foreign objects (such as dirt, bacteria, or cancer cells) out of the blood and into the lymphatic capillaries. Since the lymph usually contains no erythrocytes, it looks clear, rather than red-colored. But eventually, the lymphatic vessels drain their cleansed lymph into several major blood veins that flow into the top of the heart.

Immune and Lymphatic Systems: “The Best Survival Offense Is A Good Defense!” The Lymphatic System: A Shadow Circulation of the Blood

Fig. 17.1 An overview of the lymphatic circulation. BP = blood pressure.

The Reticuloendothelial System

The lymphatic system is sometimes given the alternate name of reticuloendothelial (reh- TIK -you-loh- en -do- THEE -lee-al) system. The reticuloendothelial or R-E system is a “little network” (reticul) of lymphatic vessels that are lined by endothelial ( en -doh- THEE -lee-al) cells. Endothelial cells are flat, scale-like cells that line both blood vessels and lymphatic vessels. And because these endothelial cells are so flat, lymph (and its contained dirt, bacteria, or cancer cells) are readily filtered through or between them.

Since the lymphatic (reticuloendothelial) system receives dirt, bacteria, debris, viruses, and cancer cells that have been filtered out of the bloodstream, it serves a critical role in immunity (body defense) by cleaning up the blood, and then returning the cleansed fluid back to the blood. Remember this general saying: “From the blood the lymph is formed, and back to the blood the lymph doth return.” The first lymph filtered from the bloodstream is dirty (in the sense that it often carries contaminants or foreign invaders), while the final lymph is clean (in the sense that the contaminants and foreign invaders have been removed). Therefore, in the process of filtering and cleaning the material that is leaked out of the blood capillaries, the lymphatic (reticuloendothelial) system provides immunity.

We can summarize the important inter-relationships by this equation:


Practice problems for these concepts can be found at:   Immune and Lymphatic Systems Test

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